Pulse modulators are known in the art for example in form of pulse position modulation (PPM) modulators and pulse width modulation (PWM) modulators. A PPM modulator codes a modulating signal into a two-level signal that has pulses at varying positions, while a PWM modulator codes a modulating signal into a two-level signal that has pulses of varying widths. PPM modulators and PWM modulators are utilized for instance in new transmitter architectures employed in mobile devices.
Energy efficient multistandard mobile devices require an optimized transmitter (TX) chain. Such optimized transmitter chains comprise usually high efficiency switching mode power amplifiers, which do not affect the phase of an input signal, but which are very non-linear concerning the amplitude of an input signal. Thus, the input signal of a high efficiency switching mode power amplifier should be a phase modulated constant envelope signal, as provided e.g. by a pulse modulator or a bandpass delta-sigma modulator. Switching mode power amplifiers using PPM or PWM for generating bandpass signals in a multimode mobile device transmitter have been described for instance in U.S. patent application 2003/0058956 A1. The use of a bandpass delta-sigma modulator has been described for example by A. Jayaraman in the article “Linear high efficiency microwave power amplifiers using bandpass delta-sigma modulators”, IEEE microwave and guided wave letters, vol. 8, No. 3, March 1998.
However, in the present mobile communication standards, the carrier frequencies are specified to lie in a region of 1–2 GHz. A common obstacle for the systems of both of the above cited documents is a need of a very high frequency clock, in case the specified carrier frequencies are to be supported. In order to implement a PPM modulator or a PWM modulator which supports a modulating signal in a frequency range of 1–2 GHz, usually a clock is needed which has a frequency from eight to sixteen times the carrier frequency, that is a frequency of 8–16 GHz. This required very high clock frequency is presently an obstacle for implementing a transmitter architecture for mobile devices which is based on PPM or PWM.
For illustration, a known solution to realize a digital PPM modulator 1 is presented in FIG. 1.
The modulator 1 comprises a word generator 11, which is connected to a phase accumulator 12. The phase accumulator 12 is connected to a signal input of a binary adder 13. A modulating signal is applied to another signal input of the binary adder 13. A clock signal generator 14 is connected to a clock input of the phase accumulator 12 and to a clock input of the binary adder 13. The output of the binary adder is connected to a bus branch 15, which provides the output signal of the modulator 1.
The modulator 1 operates as follows.
The word generator 11 provides a generated phase word to the phase accumulator 12. The phase accumulator 12 creates a digital sawtooth wave, the frequency of the sawtooth wave being determined by the provided phase word. The clock signal generator 14 provides a clock to the clock input of the phase accumulator 12 and defines thereby the accumulation rate of the phase accumulator 12. The generated sawtooth wave is fed to the binary adder 13 together with a modulating signal.
The modulating signal and the sawtooth wave are then added in the binary adder 13. The digital sawtooth wave cause the binary adder to overflow at periodic moments depending on the respective level of the modulating signal. The bus branch 15 is then used for choosing a single bit output from the binary adder indicating the time of this overflow. As a result, a PPM modulation is obtained. In order to achieve an acceptable modulation accuracy, the clock frequency has to be at least eighth times higher than the carrier frequency, i.e. than the frequency with which a new value of the modulating signal is provided.
A digital PPM or PWM is therefore usually not used for high carrier frequencies. Instead, the PPM or PWM is usually created in an analogue domain.
A known method to produce a PPM and a PWM in an analogue domain is illustrated in FIGS. 2a and 2b, which were taken from the document “Communication systems, An introduction to signals and noise in electrical communication”, 3 rd edition, McGRAW-HILL 1986, ISBN 0-007-100560-9, by A. B. Carlson.
FIG. 2a is a block diagram of an analog pulse position modulator. A comparator 21 comprises a first input for receiving a modulating signal x(t) and a second, inverting input to which a sawtooth generator 22 is connected. The output of the comparator 21 constitutes on the one hand a first output of the modulator. On the other hand, the output of the comparator 21 is connected via a monostable 23 to a second output of the modulator.
FIG. 2b presents three diagrams with signals occurring in the modulator of FIG. 2a. An upper diagram shows an analog modulating signal x(t) input to the first input of the comparator 21 over time t. In addition, the upper diagram shows an analog sawtooth waveform over time t, which is generated by the sawtooth generator 22 and provided to the second, inverting input of the comparator 21.
The comparator 21 compares the received signals to produce the sequence of PWM pulses shown in a middle diagram of FIG. 2b over time t. In the figure, the PWM pulses are referred to as pulse density modulation (PDM) pulses. More specifically, a respective pulse has a rising edge when the sawtooth waveform has a falling edge, and a falling edge when the amplitude of the subsequent rising flank of the sawtooth waveform becomes equal to the modulating signal. In case PPM pulses are desired instead of PWM pulses, the generated PWM pulses are fed to the monostable 23, which produces equal length pulses whenever the comparator 21 detects that a rising flank of the sawtooth waveform becomes equal to the modulating signal, i.e. whenever there is a falling edge of a PWM pulse. The resulting PPM pulses are shown in a lower diagram of FIG. 2b over time t.
In this analog solution, the frequency of the sawtooth waveform has to be about eight times higher than the modulation frequency.